Poliomyelitis is an acute infection of the central nervous system cause by poliovirus. Poliovirus is an important human pathogen, yet our understanding of how the virus causes disease is incomplete. The goal of the proposed studies is to study the pathogenesis of poliomyelitis in transgenic mice that synthesize the cell receptor for poliovirus, PVR. This goal will be pursued through the following specific aims. 1. Determine how poliovirus replicates in the presence of IFN. Poliovirus replicates in cultured cells treated with IFNa A single amino acid change in the viral 2Apno proteinase renders the virus sensitive to IFNa without affecting inhibition of host cell translation. Experiments are designed to provide evidence that 2Apro proteinase is essential for growth in the presence of IFNa, and to determine whether 2Apro blocks induction of IFN synthesis, or the induction of ISGs by IFN. We have also found that poliovirus infection blocks the induction of ISGs. We will determine whether this effect is caused by viral proteinase, 2Apro or 3Cpro, and identify the steps in innate sensing and IFN induction that are affected by viral replication.2. Determine the relationship between the IFNo/p response and poliovirus pathogenesis in mice. The restricted tropism of poliovirus in CD155Tg mice is determined by the IFNo/p response. Non-neural organs of CD155Tg mice appear to be protected from poliovirus infection by the induction of a vigorous ISG response. In contrast, the brain and spinal cord do not mount an ISG response and are not protected from poliovirus infection. These important observations lead to a number of other questions about the relationship between poliovirus pathogenesis and the IFNa/p response, which are addressed in this aim. Why do mice mount a poor ISG response to poliovirus in the central nervous system? What is the extent of the ISG response to poliovirus in the mouse intestine, and is it responsible for preventing infection at that site? Are the Sabin vaccine strains neurovirulent in CD155Tg x IFNAR*'* mice? Does reduced replication of these strains in the central nervous system lead to an enhanced ISG response that limits viral replication? 3. Determine the role of pattern recognition molecules in poliovirus replication and pathogenesis. Innate responses to viral infection are triggered when cellular pattern recognition molecules engage viral macromolecules. Experiments in this specific aim are designed to determine the role of specific pattern recognition molecules in poliovirus replication and pathogenesis. We will determine the roles of TLR3, TLR7/8, RIG-I, and MDA-5 in sensing poliovirus replication in mice and in cultured cells. The biological role of poliovirus-induced cleavage of RIG-I and MDA-5 will be determined. Because poliovirus is a model pathogen, the results will contribute to the control of other medically relevant viruses.